From the reasons of high chemical stability, low price, excellent balance of physical properties, possible recycling, etc., the amount of using polyolefinic resins such as polypropylene is increasing centering around automotive parts, household electrical appliances and moldings for general goods.
However, polyolefinic resin is nonpolar and has a drawback of difficult painting and adhesion. Hence, it is a general trend to use chlorinated polypropylene or acid-modified chlorinated polypropylene as a constituting element of primer, paint, ink and adhesive.
Traditionally, these chlorinated resins were used after dissolving into aromatic organic solvent such as toluene or xylene, but, from the viewpoints of environmental problems and safety and hygiene, attempts on aqueous conversion are being made widely (Japanese Patent Publication No. Hei 8-6009, Japanese Unexamined Patent Publication Nos. Hei 5-209006 and Hei 6-80738, Japanese Patent No. 2769958, WO90/12656, etc.).
However, these aqueous converted resins have such a problem that they require a great deal of energy and time in the processes of drying and baking after coated onto substrate over the solvent-based resins.
For solving this problem, correspondences to high solidification and low-temperature baking of aqueous dispersion of chlorinated resins have become to be sought. On the other hand, high modularization of polyolefin substrates is advancing and, in particular, under the low-temperature baking condition sat around 80 to 90° C., sufficient adherent strength cannot be achieved with the aqueous dispersions that originate from conventional chlorinated resins, resulting in difficult correspondence. Furthermore, in the uses for automotive parts, it has become to be sought to combine also with gasohol resistance etc., creating a situation of more difficult correspondence.
One of the means for corresponding to the low-temperature baking, a method of lowering the softening temperature of raw material polypropylene is effective, but, in order to lower the softening temperature in the polymerization process that uses conventional Ziegler·Natta catalyst, it is required to increase the composition ratio of ethylene or other α-olefin, resulting in decreased physical properties such as adherence and gasohol resistance. If increasing the molecular weight of chlorinated resin to suppress the decreased physical properties, then poor dispersion results due to increased melt viscosity in the aqueous converting process, or the viscosity of final product obtained increases, thus becoming unsuitable for the high solidification.
Moreover, conventional polypropylene and copolymer of propylene with ethylene or other α-olefin have a wide molecular weight distribution, hence the molecular weight distribution after acid modification or chlorination also becomes wide, and the adherence and solvent resistance decrease due to the existence of relatively low-molecular components. In particular, decrease in the gasohol resistance is remarkable, and, in order to improve this, a process for removing the low-molecular components is required by performing extraction with solvent, etc., which is uneconomical.
Whereas, syndiotactic polypropylene (hereinafter SPP), produced by using metallocene catalyst has features of low softening temperature and also narrow molecular weight distribution, and technologies of primer comprising chlorinated resin that utilizes this SPP are disclosed (Japanese Patent No. 3045498 etc.). However, since most of the substrates have isotactic polypropylene (IPP) produced by using Ziegler·Natta catalyst as a major component, sufficient adherent strength cannot be achieved.
As described above, with the aqueous dispersions that use conventional chlorinated resins, it was impossible to achieve good adherent strength and gasohol resistance, while corresponding to the high solidification and low-temperature baking.
The purpose of the invention is to provide a novel aqueous dispersion that allows the high solidification and that has good adherence and gasohol resistance to polypropylene and various hard-adherent resin substrates even on low-temperature baking.
As a result of diligent investigations to solve the subjects, the inventors have found that an aqueous dispersion of carboxyl group-containing chlorinated propylenic random copolymer originating from propylenic random copolymer polymerized in the presence of metallocene catalyst solves the subjects. In addition, it has been found that the aqueous dispersion of the invention not only exhibits good adherence on to polypropylene substrates, but also exhibits good adherence similarly onto PET and other hard-adherent substrates.